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Processes - Ferrous (Iron / Steel): Coating / Finishing / Annealing

After casting and reheating, the semi-finished slabs, billets, and other shapes are further processed into plate, bar and structural products as shown in the steelmaking process diagram graphic. Coating, finishing and annealing steps are performed on many of these products. The diagram on the following page indicates the finishing processes for steel sheet, a typical steel product. Generally, these process steps (forming, cleaning, annealing) are also used for other shapes.

Go to the Steelmaking Process Diagram

Depending on the product shape and grade, some or all of these processes may be completed. As the forming and finishing diagram (on the previous page) illustrates, fuel use is common for reheating, pickling (steam heating or immersion heating systems are common), and annealing (completed in either batch or continuous furnaces, typically using a protective atmosphere).

The following list and table illustrate the fuel and energy use for common processes. The source document is a bit misleading as it does not show fuel use for cleaning and annealing. However, it's sufficient in other areas to be of value. Generally, the fuel consumption for batch and continuous annealing is in the range of 0.5 to 1.2 Mcf / ton (averages will vary based on the process and age of equipment with the higher use generally being attributed to the batch anneal equipment).

Key Energy Facts - Forming and Finishing - Energy Use per Net Ton of Product

  • Reheat Furnace - 1.6 106 Btu average; 1.4 106 Btu for modern furnaces
  • Hot Rolling - 0.8 106 Btu
  • Acid Pickling - 1.2 106 Btu
  • Cold Rolling - 0.7 106 Btu
  • Cleaning / Annealing - 1.0 106 Btu

After the initial annealing step (some products require multiple rolling and annealing), the products are further processed based on their basic shape and end-use. Two brief examples are provided (many more exist).

  • Sheet coils are shipped direct (to purchasers for stamping, or further rolling and annealing steps) or are sent to the galvanizing (or other coating) shop in a primary steel facility.
  • Billets are formed into bars, annealed, sometimes cut to length, and shipped to customers.

The following general statement applies to the steel industry (although exceptions can be noted). Integrated primary steel producers will typically complete additional coating, finishing, and annealing processes. While "Mini-mills" usually produce a bar, strip or other product for shipment to others (to complete any necessary processing).

The processes used to coat or further finish these materials include the following:

  • Galvanizing - Both "Hot-Dip" and electrogalvanize. Products to be coated are first cleaned and dried prior to coating. Some continuous hot-dip processes utilize radiant tube burners and protective atmosphere to reduce surface oxides (clean the product) prior to coating. Galvanize materials consist of molten pure zinc that form an alloy with the iron on the surface of the product being coated.
  • Terne line coating - Sheet and plate is dipped into a molten bath consisting of 80% lead and 20% tin.
  • Other organic coatings (paint, varnish, lacquers, resins, etc) are used on products as well. However, in many cases, these process coating steps are completed by customers of the primary steelmakers.